Explanation of Passivhaus

20 Sept: DNC writes: The Isover website has a very succinct explanation of the principles of the Passivhaus, and to avoid the risk of upsetting someone at the Passivhaus institute, they call it the Multi Comfort house. There are good diagrams, and some downloadable PDFs.
   It is impossible for an existing British house to be brought up to the standard of Passivhaus as the requirements for insulation extend to the tiniest levels of detail, such as avoiding cold bridges at every conceivable junction. No matter how well insulated your main surfaces are, there are heat losses at the junctions, eg. roof to wall. Traditional houses, and ones being built have cold bridges all over the place. Also, British houses are naturally ventilated, and Passivhaus should mostly be nearly airtight, with a heat recovery ventilation system.
  In a well insulated house, the ventilation in winter becomes the main cause of heat loss. It is difficult to build an airtight house, let alone persuade a British family to live in one and operate it correctly.

So what is the next best thing you can do? Well, the first thing is to INSULATE! 
Note: External insulation is vastly more effective than Internal.
The cost of getting it applied externally is mitigated by the benefit of leaving your interior unchanged in size and appearance. It is better because the thermal mass of your house remains within the shell, acting as a conditioner of the air temperature. It will cover a large number of the thermal bridges. External insulation works better if you take it below ground level, with just a metal expansion joint along the line of the old damp proof course.
If you have a garage up against your house, it is an easy DIY job to stick insulation to the external wall within the garage, not requiring to be weather proofed.

The other aspect is to check your windows, and consider a programme of window replacement with modern panes and frames with thermal breaks in them. Do not consider expensive options like mechanically assisted heat recovery ventilation until you have made the house more airtight and better insulated.